1. Field of the Invention
This invention relates to an AC amplifier. More particularly, the invention relates to an AC amplifier which is useful when applied as an AC amplifier of the CMOS type composed of a PMOS transistor and an NMOS transistor.
2. Description of the Related Art
FIG. 9 is a circuit diagram showing a basic AC, amplifier of the CMOS type. In this AC amplifier, as shown in this drawing, a PMOS transistor T1 and an NMOS transistor T2 are connected in series. Both gates of transistors T1 and T2 are connected together and used as an input terminal IN, while both drains of transistors T1 and T2 are connected as an output terminal OUT. A feedback resistance Rf1 is connected between the input terminal IN and the output terminal OUT to constitute a self-bias circuit.
In order to decrease minimum operating voltage VDDmin in the above-described AC amplifier, it is necessary to decrease both of the threshold voltage Vtp of the PMOS transistor T1 and the threshold voltage Vtn of the NMOS transistor T2. Since n-type polysilicon is use as gate electrode in an ordinary MOS transistor, if the threshold voltage Vtp of the PMOS transistor is 0.4V or less, the off-state leakage current of the PMOS transistor sharply increases. Generally IC's of low-voltage operation are demanded low current consumption, and the use of a device involving a large off-state leakage current is not a right choice.
The threshold voltage Vtn of the NMOS transistor can be decreased to 0.35V. However, it is still difficult to decrease the minimum operating voltage VDDmin to 1.0V or less.
In an AC amplifier used for a crystal oscillator, for example, there is a growing demand for decreasing the minimum operating voltage VDDmin without sacrificing the amplification factor A of the amplifier. Concretely, demand is surging for the advent of an AC amplifier having a minimum operating voltage VDDmin of 1.0V or less.
An AC amplifier shown in FIG. 10 can achieve a low voltage operation without lowering the threshold voltage of MOS transistors, and has a required amplification factor A.
In the AC amplifier shown in FIG. 10, the gate of its PMOS transistor T1 is biased by a bias resistance Rb and a reference voltage source VB1. The bias voltage Vb1 of the reference voltage source VB1 is set to be greater by an excess voltage Xp than the threshold voltage Vtp of the PMOS transistor T1. Hereinbelow, the bias voltage Vb1 is expressed as Vtp+Xp.
In this AC amplifier, a capacitor Cc1 is inserted between the gate of the PMOS transistor T1 and the gate of an NMOS transistor T2. Thus, different bias voltages can be applied to the respective gates, but both gates are short-circuited with respect to high frequency AC signal. The gate and drain of the NMOS transistor T2 are connected via a feedback resistance Rf1.
FIGS. 11A and 11B show voltage-current characteristics when the AC amplifier shown in FIG. 10 is operated at 0.9 V. FIG. 11A is a characteristic view showing the relation of a drain saturation current of the PMOS transistor T1 with respect to the gate voltage Vg and a drain saturation current of the NMOS transistor T2 (Vg is based on ground potential). The current curve L7 in FIG. 11A shows that a drain current Id1 can flow out from the PMOS transistor T1 when the bias voltage Vtp+Xp is applied to the gate of the PMOS transistor T1. The current curve L8 in FIGS. 11A and 11B shows the drain saturation current of the self-biased NMOS transistor T2 with respect to the output voltage Vout (Vg=Vout in self biased NMOS transistor T2).
With the AC amplifier of the basic structure shown in FIG. 9, at the operating point B1, the PMOS transistor T1 and the NMOS transistor T2 both act in their saturation region. Thus, in order for the AC amplifier of FIG. 10 to have the same amplification characteristic as the AC amplifier shown in FIG. 9 the PMOS transistor T1 and the NMOS transistor T2 both need to act in the saturation region. Here, the NMOS transistor T2 whose gate and drain are connected by the feedback resistance Rf1 has a gate-source voltage and a drain-source voltage identical with each other, and its threshold voltage Vtn is positive. Thus, the NMOS transistor T2 works always in the saturation region.
A characteristic curve L9 shown in FIG. 11B is a drain saturation current of the PMOS transistor T1. In order for the PMOS transistor T1 to act in the saturation region, the drain-source voltage of the PMOS transistor T1 has to be greater than a non-saturation region voltage Yp, as shown in FIG. 11B. Here, the non-saturation region voltage Yp is a drain voltage Vd necessary for the saturation of the drain current Id when the bias voltage (Vtp+Xp) is applied between the gate and the source of the PMOS transistor T1. Generally, the relation Yp≦Xp holds. Let the drain-source voltage of the NMOS transistor T2 be expressed as (Vtn+Xn) when a drain saturation current flows in the PMOS transistor T1. In this case, the drain-source voltage of the PMOS transistor T1 is expressed with Vdsp in FIG. 11A. Thus, the followings equation (1) holds:VDD=Vdsp+(Vtn+Xn)   (1)On the other hand, power supply voltage VDD, which permits the AC amplifier to perform a sufficient amplifying operation, is represented by the following equation (2) based on Vdsp≧Yp:VDD≧Vtn+Xn+Yp   (2)Thus, the minimum operating voltage VDDmin of the AC amplifier is defined by (Vtp+Xp) or (Vtn+Xn+Yp), whichever is higher in value. That is, the minimum operating voltage VDDmin=MAX (Vtp+Xp, Vtn+Xn+Yp). If Vtp equals Vtn, and Xn equals Xp, then the minimum operating voltage VDDmin=Vtn+Xn+Yp. Here, a comparison will be made between the minimum operating voltage VDDmin of the AC amplifier shown in FIG. 9 and that of AC amplifier shown in FIG. 10. The comparison shows that the minimum operating voltage VDDmin of the AC amplifier shown in FIG. 10 can be decreased, at least, by a voltage corresponding to the threshold voltage Vtp of the PMOS transistor T1.
Japanese Patent publication No. 1986-17406 can be named as an example of related art which has the same idea as that of the AC amplifier shown in FIG. 10 and which can attain the same object as that of this AC amplifier. The AC amplifier shown in Japanese Patent publication No. 1986-17406 is equivalent to that of FIG. 10 in which the reference voltage source VB1 is put to zero and the bias resistance Rb is directly grounded.
It is not an exaggeration to say that a crystal, oscillator, which is a typical circuit utilizing an AC amplifier, is currently used for most of electronic equipment. Generally, the crystal oscillator needs a high amplification factor A at the start of oscillator thus requiring a large drain current Id1 at the operating point. In a stable oscillation state, on the other hand, a large current at the operating point results in a wasteful through current. This is a major problem from the viewpoint of low power consumption.
In FIG. 10, when an increasing AC signal of a cycle T as shown in FIG. 12 is given to its input terminal IN, a through current of the AC amplifier Id fluctuates as shown in FIG. 13. A line L4 in FIG. 13 represents a drain current Id1 when the gate of the PMOS transistor T1 is biased with Vb1 by the reference voltage source VB1. That is, the bias voltage on the gate of the PMOS transistor T1 is fixed, whereby the drain current Id1 at the operating point B1 as shown in FIG. 1A flows as a through current every half of the signal cycle T. Accordingly, with the AC amplifier concerned, the great drain current Id1 required for start of oscillation flows periodically, thus making it difficult to decrease the current consumption of the AC amplifier.
With the AC amplifier concerned, moreover, the operating point B1 greatly leans toward the power supply voltage VDD. Thus, the duty (the ratio of the time during which a waveform is at a high level to the cycle of the waveform) of the output AC signal greatly deviates compared with the input AC signal. To remove this deviation in duty, it is necessary to add a new correction circuit or the like.
The present invention has been accomplished in the light of the above-mentioned problems with the earlier technologies. It is an object of the present invention to provide an AC amplifier of low voltage and low current operation which minimizes a deviation in the duty of an output signal waveform, and has a sufficient amplification factor.